Flexible electronics is a technology that attaches inorganic/organic devices to flexible substrates to form circuits. Compared with traditional silicon electronics, flexible electronics refers to thin-film electronic devices that can be bent, folded, twisted, compressed, stretched, and even deformed into arbitrary shapes while still maintaining high-efficiency optoelectronic performance, reliability, and integration.
Countries such as the United States, Japan, and South Korea have strategically deployed flexible electronics projects, which will maintain a high-speed growth trend in the high-tech field for a long time, and it is also a historical opportunity that China should seize as much as possible.
Materials commonly used in flexible electronics
1. Flexible substrate
In order to meet the requirements of flexible electronic devices, properties such as thinness, transparency, flexibility and stretchability, insulation and corrosion resistance have become key indicators of flexible substrates.
Common flexible materials are: polyvinyl alcohol (PVA), polyester (PET), polyimide (PI), polyethylene naphthalate (PEN), paper, textile materials, etc.
Polyimide material has the advantages of high temperature resistance, low temperature resistance, chemical resistance and good electrical properties, and is the most potential material for flexible electronics. In the selection of flexible substrates, in addition to the characteristics of high temperature resistance, the light transmittance, surface roughness and material cost of flexible substrates are all factors that must be considered.
Polydimethylsiloxane (PDMS) is also a widely recognized flexible material. Its advantages include convenience, easy availability, stable chemical properties, transparency and good thermal stability. Especially under ultraviolet light, the characteristics of the adhesive area and the non-adhesive area make it easy to adhere to the surface of electronic materials.
Although the conversion temperature of PET is low, about 70~80°C, PET is cheap and has good light penetration. It is a material with high cost performance for transparent conductive films.
2. Metal material
Metal materials are generally conductive materials such as gold, silver and copper, and are mainly used for electrodes and wires. For modern printing processes, conductive nano-inks are mostly used as conductive materials, including nanoparticles and nanowires. In addition to good electrical conductivity, metal nanoparticles can also be sintered into films or wires.
3. Organic materials
Large-scale pressure sensor arrays are very important for the development of future wearable sensors. Pressure sensors based on piezoresistive and capacitive signal mechanisms have signal crosstalk, resulting in inaccurate measurements, which has become one of the biggest challenges in the development of wearable sensors.
The use of transistors offers the possibility of reducing signal crosstalk due to their perfect signal conversion and amplification properties. Therefore, much research in the fields of wearable sensors and artificial intelligence revolves around how to obtain large-scale flexible piezotransistors.
The p-type polymer materials traditionally used in field effect transistor research are mainly thiophene polymers, and the most successful example is the poly(3-hexylthiophene) (P3HT) system. Naphthalene tetraimide and perylene tetraimide show good n-type field effect properties, are the most widely studied n-type semiconductor materials, and are widely used in small molecule n-type field effect transistors.
4. Inorganic semiconductor materials
Inorganic semiconductor materials represented by ZnO and ZnS have shown promising application prospects in the field of wearable and flexible electronic sensors due to their excellent piezoelectric properties.
The electronic energy band of piezoelectric ZnS is tilted due to the voltage effect under pressure, which can promote the excitation of manganese ions, and the subsequent de-excitation process emits yellow light.
5. Carbon material
Carbon materials commonly used in flexible wearable electronic sensors include carbon nanotubes and graphene. Carbon nanotubes have the characteristics of high crystallinity, good electrical conductivity, large specific surface area, micropore size can be controlled by synthesis process, and specific surface utilization rate can reach 100%.
Graphene has the characteristics of light, thin, transparent, and good electrical and thermal conductivity. It has extremely important and broad application prospects in sensing technology, mobile communication, information technology and electric vehicles;
In the application of carbon nanotubes, the conductive polymer sensor obtained by compounding multi-armed carbon nanotubes and silver and obtained by printing, under 140% stretching, the conductivity is still as high as 20S/cm.
When carbon nanotubes and graphene are combined, highly stretchable transparent field-effect transistors can be fabricated. It combines graphene/single-walled carbon nanotube electrodes and single-walled carbon nanotube mesh channels with wrinkled inorganic dielectric layers. Due to the wrinkled alumina dielectric layer, there is no drain current change under more than one thousand stretch-relaxation cycles of 20% amplitude, showing good sustainability.
Application fields of flexible electronics
1. Flexible electronic display
Flexible electronic display is a brand-new product developed on the platform of flexible electronic technology. It is made of flexible materials and can be deformed and bendable. At present, flexible display modes (e-paper technology, LCD, OLED, etc.) can be realized to make display devices on flexible substrates, such as writable e-books, U disk capacity displays, etc.
2. Flexible energy storage
Flexible energy storage is an emerging energy storage technology that uses organic/inorganic material electronic devices on flexible/ductile plastic or thin metal substrates. Medical, national defense and other fields have broad application prospects, and have been successfully used in flexible electronic displays, organic light-emitting diodes (OLEDs), printed RFID, thin-film solar panels, and surface-mounting for electronics.
For example, a small foldable 210 mAh battery made by Samsung can be used in wearable devices. The thickness of the battery itself can be as thick as 0.3 mm, and it can be bent and folded 50,000 times on the human wrist. No any glitches occur.
3. Flexible medical electronics
The basic feature of flexible medical electronics is to integrate various electronic components on a flexible substrate to form a skin-like flexible circuit board, which has high flexibility and elasticity like skin.
Flexible medical electronics can be naturally integrated with human tissues for a long time, and can accurately measure medical indicators, such as body temperature, respiration, blood pressure, ECG, etc., and provide real-time basic data for big data medical treatment.
4. Flexible circuit board
Flexible Printed Circuit (FPC) is a highly reliable and excellent flexible PCB made of polyimide or polyester film as the base material. It has the characteristics of high wiring density, light weight, thin thickness and good bendability, which perfectly fits the main theme of development of thinner and smaller.
The FPC industry is dominated by Japan, the United States and South Korea. In recent years, the increase in production costs has prompted the FPC industry to gradually shift its focus to the Chinese market. FPC is located in the middle and upper reaches of the electronics industry chain. Its direct raw material upstream is flexible copper clad laminate FCCL, and its downstream is terminal consumer electronics products.
At present, Japanese-funded enterprises occupy a dominant position in the upstream of the industrial chain with the first-mover advantage, but it started relatively late in China and is relatively weak.
In recent years, the flexible electronics market has expanded rapidly and has become a pillar industry in some countries. It has broad application prospects in information, energy, medical, national defense and other fields.
According to IDTechEX forecast, the global flexible electronics market will be US$46.94 billion in 2018 and US$301 billion in 2028. It is the commanding height of technology that various countries are vying to seize, and it is also an opportunity that we should pay attention to and seize.